American Champion Aircraft 7ECA N798L incident near Longview, Texas

Probable cause

The failure of the lower hinge barrel due to fatigue cracking. Contributing to the failure of the lower hinge barrel was increased stress from end face contact likely associated with worn bushings or displaced needle bearings.

NTSB analysis

After the uneventful instructional flight landed, the airplane was shut down on the ramp. A second flight instructor and student pilot then completed a preflight inspection before their planned instructional flight. During their preflight inspection, the lower hinge for the rudder was found fractured from the vertical spar tube.

Examination of the rudder and the lower hinge attachment hardware revealed the lower hinge barrel fractured from fatigue cracking that initiated along the toe of the weld that joined the hinge barrel and the rudder post. Cracking started on both the outer and inner surfaces of the lower rudder post, propagating into the post cross section, consistent with reverse-bending fatigue. The inner surface cracking initiated from small corrosion pits on the inside surface, and the outer surface fatigue cracking initiated along geometrical features at the weld toe that acted as stress concentration sites. The fatigue cracking propagated from the top down the left and right sides until the final overstress detached a small downward-facing ligament of the post at an unknown time.

A black protective coating had been applied to the frame of the rudder post. This layer was consistent with the prescribed black powder coating from manufacture. A successive series of paint layers were present on top of this black coating: white, red, white, and then blue on the exterior. The white layers were likely primer.

The blue and white paint layers covered areas of the post where cracks had developed. However, worn surfaces on the hinge knuckles were also covered with blue paint, indicating the rudder post had operated for some time before the paint was applied. The presence of corrosion pitting suggests that the paint and other coatings were either not sufficient to protect the underlying alloy surfaces or were compromised during the rudder’s life.

The rudder is expected to accommodate the variable loads that stress the upper and lower hinge knuckle attachments during normal operation. However, the contact that produced wear at the hinge barrels could have contributed to elevated stress at the attachment locations. The vertical stabilizer carries vertical and lateral loads from rudder accelerations and surface pressures through the upper and lower hinges.

The weld toes between the hinge barrels and the rudder post would concentrate these variable stresses at this location; additionally, both hinge barrels' lower surfaces were worn, indicating prolonged surface contact with an associated increase in stress. While fatigue cracking had initiated at both the inner and outer surfaces of the rudder post, the extent of crack growth propagating from the exterior surface indicates the primary initiation area was along the toe of the weld.

According to the FAA, where the airplane was based is classified as an area of having moderate corrosive exposure for aircraft structures.

Based on the available evidence, the lower hinge barrel likely failed due to fatigue cracking at an unknown time. Contributing to the failure of the lower hinge barrel was increased stress from end face contact likely associated with worn bushings or displaced needle bearings.

After the incident, the operator and the airframe manufacturer both expanded their inspection requirements for these areas by using magnification glass visual and dye penetrant inspections, which will likely enhance future crack detection by maintenance personnel.

Text from the NTSB. Open the NTSB docket for CEN25LA086 for the full record.